You can get great OS site-centred maps on lightweight waterproof paper from these guys:
Based in Hope so you're supporting a local business. You can select the map area on the screen so you can see exactly what's included. I've had one tweaked for the area around my house for the last four years or so and it's still going strong with regular use. It's a lightweight, tough, waterproof paper construction, not a heavy laminate, so no heavier than a conventional paper map and probably lighter in fact.
Oh, the web site's not the most beautiful thing in the world, but the maps are good.
And the plus point over an Ortlieb map case is that you simply use it like a normal map, no need to rearrange the map in the case when you ride off the edge.